If someone is mulish, they refuse to do a thing, or refuse to agree, in a way that feels unreasonable — they dig in, like a mule that will not take a step. It is about stubborn will, not about looking like the animal. The word is a little old-fashioned and a little bookish; in speech, most people just say stubborn. You might still meet mulish in stories, in a teacher's complaint, or when a writer wants a sharper, slightly amused picture of someone who will not be moved. Though actually, stubborn can be mild (“I was stubborn about the time”); mulish almost always sounds like a criticism. It paints the person as unreasonably fixed, not bravely firm.

像騾子一樣固執、死不讓步（偏書面；口語多半直接說固執）